In vitro stimulation of influenza virus-immune cytotoxic effector cells in human peripheral blood leukocyte (PBL) populations; and (b) measurement of effector cell activity on virus-infected cryopreserved human PBL target cells was investigated. Effector cells generated in vitro are influenza type-specific and are HLA-A and HLA-B restricted. The cytotoxic response appears to be controlled by HLA-linked immune response (Ir) genes since: (a) cytotoxic activity is predominantly associated with antigens of only one haplotype; and (b) effector cells from certain donors recognize virus in association with some but not all self HLA-A and -B antigens. This Ir gene control is HLA-linked and antigen-specific, since preferential recognition among HLA-identical siblings is different for influenza A and B viruses. Comparative studies of HLA-A positive and of HLA-A3 positive donors for these CTL responses indicate that greater heterogeneity of these HLA antigens can be demonstrated by CTL than by current serology. The CTL response to influenza virus was found to depend on interacting populations of helper and CTL precursor T cells.